Phosphatases remove phosphate groups from molecules previously activated by kinases and control most cellular signaling events that regulate cell growth and differentiation, cell-to-cell contacts, the cell cycle and oncogenesis. Protein phosphorylation is the ubiquitous strategy used to control the activities of eukaryotic cells. It is estimated that more than 1000 of the 10,000 proteins active in a typical mammalian cell are phosphorylated. In phosphorylation, the high energy phosphate which confers activation is transferred from adenosine triphosphate molecules to a protein by protein kinases, and is subsequently removed from the protein by protein phosphatases.
There appear to be three evolutionarily-distinct protein phosphatase gene families: protein phosphatases (PPs); protein tyrosine phosphatases (PTPs); and acid/alkaline phosphatases (APs). PPs dephosphorylate phosphoserine/threonine residues and are an important regulator of many cAMP-mediated hormone responses in cells. PTPs reverse the effects of protein tyrosine kinases and play a significant role in cell cycle and cell signaling processes. APs dephosphorylate substrates in vitro, although their role in vivo is not well known.
PPs may be cytosolic or associated with a receptor and can be separated into four distinct groups: PP-I, PP-IIA, PP-IIB, and PP-TIC. (Cohen, P. (1989) Annu. Rev. Biochem. 58:453-508.) PP-IIC is a relatively minor phosphatase that is unrelated to the other three. The three principle PPs are composed of a homologous catalytic subunit coupled with one or more regulatory subunits. PP-I dephosphorylates many of the proteins phosphorylated by cylic AMP-dependent protein kinase (PKA) and is an important regulator of many cyclic AMP-mediated hormone responses in cells. PP-IIA has broad specificity for control of cell cycle, growth, and proliferation, and DNA replication, and is the main phosphatase responsible for reversing the phosphorylations of serine/threonine kinases. PP-IIB, or calcineurin (Cn), is a Ca.sup.+2 activated phosphatase and is particularly abundant in the brain.
PTPs remove phosphate groups from selected phosphotyrosines on particular types of proteins. In so doing, PTPs reverse the effects of protein tyrosine kinases (PTK) and play a significant role in cell cycle and cell signaling processes. (Charbonneau, H. and Tonks, N. K. (1992) Annu. Rev. Cell Biol. 8:463-493.) PTPs possess a high specific enzyme activity relative to their PTK counterparts. In the process of cell division, for example, a specific PTP (M-phase inducer phosphatase) plays a key role in the induction of mitosis by dephosphorylating and activating a specific PTK (CDC2) leading to cell division. (Krishna, S. et al. (1990) Proc. Natl. Acad. Sci. 87:5139-5143.) Tyrosine phosphorylations are therefore short lived and uncommon in resting cells.
Many PTKs are encoded by oncogenes, and it is well known that oncogenesis is often accompanied by increased tyrosine phosphorylation activity. It is therefore possible that PTPs may serve to prevent or reverse cell transformation and the growth of various cancers by controlling the levels of tyrosine phosphorylation in cells. This is supported by studies showing that overexpression of PTP can suppress transformation in cells and that specific inhibition of PTP can enhance cell transformation. (Charbonneau and Tonks, supra.)
PTPs are found in transmembrane, receptor-like and nontransmembrane, non-receptor forms, and are diverse in size (from 20 kDa to greater than 100 kDa) and structure. All PTPs share homology within a region of 240 residues which delineates the catalytic domain and contains the common sequence VHCXAGXXR near the carboxy terminus. The combination of the catalytic domain with a wide variety of structural motifs accounts for the diversity and specificity of these enzymes. In nonreceptor isoforms, noncatalytic sequences may also confer different modes of regulation and target PTPs to various intracellular compartments.
The discovery of new protein phosphatase-related molecules and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of cancer and immune and reproductive disorders.